1. Field of the Invention
This invention relates to a control system for a general-purpose engine.
2. Description of the Related Art
The general-purpose engine is a well-known spark-ignition internal combustion engine with not more than two cylinders that sucks into the cylinders, ignites and burns an air-fuel mixture produced in a carburetor by mixing gasoline fuel and an amount of intake air regulated by a throttle valve. General-purpose engines are used as power sources for portable generators, agricultural machines, civil engineering equipment and various other kinds of machinery.
Since general-purpose engines of this type are desirably rugged and inexpensive, they use a carburetor-type fuel supply system and are started manually with a recoil starter. As they are intended for use in a fixed engine speed range, their speed is usually controlled using a mechanical governor comprising weights and a spring.
Still, even in this type of general-purpose engine, the recent trend in development is toward introduction of PID control of the throttle valve using a linear solenoid, stepper motor or other actuator connected to the throttle valve and a microcomputer-based electronic control unit (ECU) for producing the actuator command values.
Further, while not for general-purpose engines but for vehicle internal combustion engines, Japanese Laid-open Patent Application No. 10(1999)-103131, for example, teaches a technology for controlling air-fuel ratio using an adaptive controller.
Although the mechanical governor is cheap because it does not need an electric power supply, it has difficulty maintaining a constant engine speed irrespective of the magnitude of the load and requires the characteristic of the spring to be set in accordance with the engine type and/or the engine speed range during use. Moreover, when an actuator is connected to the throttle valve and the actuator command values are determined using a PID control law, the PID control gain has to be set according to the load such as the generator and some similar parameters. Then when the utilized engine speed range is changed, the gain has to be reset. In other words, when control is conducted using a PID control law, optimum stability and tracking property is not ensured when a characteristic of the subject of control (plant) changes.
In contrast, when actuator command values are set using an adaptive control law, the amount of computation increases, but, owing to the fact that the gain can be set without taking load into account, robust control can be achieved with respect to changes in a characteristic of subject of control (plant). Another advantage is that the utilized engine speed can be set freely.
Thus, the application of the adaptive control to such a general-purpose engine has long been desired.
Further, when such adaptive control is applied to an actual general-purpose engine, overshooting of the desired value or control hunting is liable to occur owing to the fact that response to sudden step-like changes in the desired value is impossible because the input value is limited by the throttle opening limit and, further, that the fuel control responsivity or response is low because of the operational delay of the fuel supply system carburetor.
Further, when such adaptive control is applied to an actual general-purpose engine, since the throttle valve of the actual engine has physical upper and lower limits, when a computed command value becomes out of the limits, the control is made impossible.
Further, when such adaptive control is applied to an actual general-purpose engine, since this type of engine has a single or two cylinders, it is difficult to build a stable control system because the engine speed (indicative of the behavior of the engine to be controlled) is liable to fluctuate markedly under the influence of the combustion cycle composed of intake, compression, expansion and exhaust strokes.
Furthermore, when such adaptive control is applied to an actual general-purpose engine, if a gain (that determines a convergence or identification speed of the adaptive controller) is set high, the engine speed would be unstable near the desired engine speed when suffered from a disturbance. On the other hand, when the gain is set low, the responsivity of control would be degraded when the characteristic of the plant (engine) fluctuates due to the change in load or some similar factors.